Fix logic for retrying nuget.exe download with Python.
Add support for HOST_PYTHON variable.
Clear internal environment variables used in find_python.bat
Use HOST_PYTHON as the actual Python if it is recent enough.
Adds HOST_PYTHON variable to AppVeyor configuration

* bpo-30450: Adds alternate download approach for nuget.exe

* Whitespace fix.

Updates ssl and tkinter projects to use pre-built externals